Waveform generating circuit, inkjet head driving circuit and inkjet recording device

ABSTRACT

An inkjet head driving circuit driving piezoelectric actuators  21  for ink ejection provided on an inkjet head H includes one D/A converter  62  that converts a digital signal into an analog voltage and outputs the analog voltage, and a waveform generating portion  64  into which an output voltage of the D/A converter  62  is input, and which generates a voltage rising waveform when the output voltage of the D/A converter  62  is larger than a predetermined potential that is midway between a maximum value and a minimum value of that output voltage, and generates a voltage rising waveform when the output voltage of the D/A converter  62  is smaller than the predetermined potential.

TECHNICAL FIELD

The present invention belongs to the technical fields relating towaveform generating circuits generating voltage waveforms, inkjet headdriving circuits driving actuators for ink ejection provided on aninkjet head, and inkjet recording devices provided with an inkjet headhaving actuators that are driven by such an inkjet head driving circuit.

BACKGROUND ART

Conventionally, inkjet heads having actuators for ink ejection are wellknown, and examples of such actuators of inkjet heads are piezoelectricactuators provided with electrodes on both sides of a piezoelectricelement, which constitute a portion of a pressure chamber accommodatingthe ink. When a pulse-shaped voltage is applied to the electrodes ofsuch an actuator, the actuator is deformed such that the volume of thepressure chamber is reduced, thus creating a pressure in the pressurechamber, which ejects ink drops from a nozzle that is in communicationwith the pressure chamber.

As shown for example in FIG. 8, the voltage waveform applied to theactuators is made of a first waveform P1′ (voltage-falling waveform), atwhich the potential falls from ground potential to the minimum potential(−Vf), a second waveform P2′ continuing the first waveform P1′ andmaintaining this minimum potential, a third waveform P3′ (voltage-risingwaveform) continuing the second waveform P2′ and rising from the minimumpotential to the maximum potential (Vf), a fourth waveform P4′continuing the third waveform P3′ and maintaining this maximumpotential, and a fifth waveform P5′ (voltage falling waveform)continuing the fourth waveform P4′ and returning from the maximumpotential to ground potential. This series of first to fifth waveformsP1′ to P5′ constitutes one driving pulse P′ for ejecting one ink dropfrom the nozzle, and the driving pulse P′ is given out repeatedly with apredetermined period.

An example of a waveform generating circuit (inkjet head drivingcircuit) generating the voltage waveform (driving pulse P′) for drivingthe actuator is shown in FIG. 9. In this drawing, numeral 101 is a CPU,which has two terminals outputting digital signals (for example of 8bits) for generating the voltage waveform. A first D/A converter 102 forconverting a digital signal into a positive analog signal and giving itout and a second D/A converter 103 for converting a digital signal intoa negative analog signal and giving it out are connected to the digitalsignal output terminals of this CPU 101. The first and second D/Aconverters 102 and 103 receive from the CPU 101 a data set signaltogether with the digital signals but from a different terminal than thedigital signals, and when this data set signal has been input and apredetermined time (data settling time) has elapsed after its input(after the output of the D/A converter 102 (or 103) has settled), theanalog voltage is given out. The first D/A converter 102 is connected toa first power source 106 giving out a positive voltage, whereas thesecond D/A converter 103 is connected to a second power source 107giving out a negative voltage.

A first and a second voltage/current converter 109 and 110 arerespectively connected to the output terminals of the first and thesecond D/A converter 102 and 103, and these first and secondvoltage/current converters 109 and 110 convert the positive and thenegative analog voltage into currents. The output terminals of the firstand second voltage/current converters 109 and 110 are connected to acurrent/voltage converter amplifier 111, which amplifies the currentsinto which the voltages have been converted by the first and secondvoltage/current converters 109 and 110, and converts the amplifiedcurrents into a voltage. It should be noted that the firstvoltage/current converter 109, which is connected to the output terminalof the first D/A converter 102, is connected to the first power source106, whereas the second voltage/current converter 110, which isconnected to the output terminal of the second D/A converter 103, isconnected to the second power source 107, and the current/voltageconverter amplifier 111 is connected to both the first power source 106and the second power source 107.

Based on the output voltage from the first and second D/A converters 102and 103, the first and second voltage/current converters 109 and 110 andthe current/voltage converter amplifier 111 generate voltage waveformslike the first to fifth waveforms P1′ to P5′. More specifically, whenthe first D/A converter 102 outputs a positive analog voltage and thesecond D/A converter 103 outputs ground potential, the voltage risingwaveform (third waveform P3′) is generated, whereas when the second D/Aconverter 103 outputs a negative analog voltage and the first D/Aconverter 102 outputs ground potential, the voltage falling waveforms(first and fifth waveforms P1′ and P5′) are generated. Furthermore, whenboth D/A converters 102 and 103 output ground potential, waveformsmaintaining the potential directly before the output of those groundpotentials (second and fourth waveforms P2′ and P4′) are generated, andthe potential between neighboring driving pulses P′ is maintained atground potential.

Then, the generated voltage waveform is applied to a multitude ofactuators of the inkjet head, through a current amplifier 113, which ismade of two transistors 113 a, and a driver IC 114. The driver IC 114includes for example switching transistors that are provided inaccordance with the actuators, and, receiving print signals from the CPU101, selects the actuators corresponding to the nozzles through whichink drops are to be ejected, thus applying the voltage waveform only tothe selected actuators.

However, this conventional waveform generating circuit necessitates twoD/A converters 102 and 103 to generate the voltage rising waveform andthe voltage falling waveform, and a positive voltage has to be suppliedto the first D/A converter 102 and a negative voltage has to be suppliedto the second D/A converter 103, thus necessitating two power sources106 and 107 for respectively outputting a positive and a negativevoltage, so that there is the problem that it is expensive and requiresrelatively much space. Furthermore, discrepancies in the characteristicsamong the first and second D/A converters 102 and 103 (discrepanciesamong variation amounts) cause discrepancies among the generatedwaveforms.

In view of these facts, it is thus an object of the present invention toimprove the configuration of the above-described waveform generatingcircuit, and thus attain a simple configuration that is less expensiveand takes up less space, and with which stable voltage waveforms can beattained.

DISCLOSURE OF THE INVENTION

In order to attain these objects, in the present invention, one D/Aconverter is provided, and when the output voltage of the D/A converteris larger than a predetermined voltage that is midway between a maximumvalue and a minimum value of that output voltage, one of a voltagerising waveform and a voltage falling waveform is generated, and whenthe output voltage of the D/A converter is smaller than thepredetermined potential, the other waveform is generated.

More specifically, according to a first invention, a waveform generatingcircuit includes one D/A converter that converts a digital signal intoan analog voltage and outputs the analog voltage, and a waveformgenerating portion into which an output voltage of the D/A converter isinput, which generates one of a voltage rising waveform and a voltagefalling waveform when the output voltage of the D/A converter is largerthan a predetermined potential that is midway between a maximum valueand a minimum value of that output voltage, and generates the otherwaveform when the output voltage of the D/A converter is smaller thanthe predetermined potential.

With this configuration, a voltage rising waveform and a voltage fallingwaveform are generated taking a predetermined potential that is midwaybetween a maximum value and a minimum value of the output voltage of oneD/A converter as a reference, so that it is not necessary to provide twoD/A converters as in conventional circuits, and moreover, one powersource outputting either a positive or a negative voltage is sufficient.Furthermore, waveform generation discrepancies due to discrepancies inthe characteristics among the two D/A converters, as in conventionalcircuits, do not occur. As a result, it is possible to achieve a circuitthat is less expensive and uses less space, and stable voltage waveformscan be generated.

According to a second invention, the first invention further includes aconstant voltage source outputting a constant voltage equal to thepredetermined potential, and a switching means, whose input can beswitched between the output voltage of the D/A converter and the outputvoltage of the constant voltage source, and which outputs one of thosetwo output voltages to the waveform generating portion, wherein theswitching means is configured such that when a digital signal forgenerating a voltage rising waveform or a voltage falling waveform withthe waveform generating portion is input into the D/A converter, theinput into the switching means is switched from the output voltage ofthe constant voltage source to the output voltage of the D/A converter.

With this configuration, the input into the switching means is set tothe output voltage of the constant voltage source when neither thevoltage rising waveform nor the voltage falling waveform are generated,and is switched from the output voltage of the constant voltage sourceto the output voltage of the D/A converter when the voltage risingwaveform or the voltage falling waveform is generated. As a result, whenneither the voltage rising waveform nor the voltage falling waveform aregenerated, a predetermined potential is output to the waveformgenerating portion from the constant voltage source, which can output aprecise voltage, even if the D/A converter outputs a voltage that isslightly different from the predetermined potential due to variations ofits characteristics, so that malfunctioning of the waveform generatingportion due to such variations in the characteristics of the D/Aconverter can be prevented.

According to a third invention, in the second invention, the switchingmeans is configured such that the input into the switching means isswitched from the output voltage of the constant voltage source to theoutput voltage of the D/A converter after the output of the D/Aconverter has settled.

That is to say, the time from the input of the data set signal until theoutput of the D/A converter has settled fluctuates depending on theoutput voltage of the D/A converter and variations in itscharacteristics, so that if there is no switching means, or even ifthere is the switching means, but the input into the switching means isswitched to the output voltage of the D/A converter before the output ofthe D/A converter has settled, then the result is variations in thegeneration timing (output timing) of the voltage rising waveform or thevoltage falling waveform by the waveform generating portion. However, inthis invention, the input into the switching means is switched to theoutput voltage of the D/A converter only after the output of the D/Aconverter has settled, so that the voltage rising waveform or voltagefalling waveform can be generated and output substantially at the sametime as the switching of the input into the switching means. As aresult, variations in the waveform generating timing brought about byfluctuations in the output settling time of the D/A converter can beprevented.

A fourth invention is an invention of an inkjet head driving circuitdriving an actuator for ink ejection provided on an inkjet head, andthis invention includes one D/A converter that converts a digital signalinto an analog voltage and outputs the analog voltage, and a waveformgenerating portion into which an output voltage of the D/A converter isinput, which generates one of a voltage rising waveform and a voltagefalling waveform and outputs it to the actuator when the output voltageof the D/A converter is larger than a predetermined potential that ismidway between a maximum value and a minimum value of that outputvoltage, and which generates the other waveform and outputs it to theactuator when the output voltage of the D/A converter is smaller thanthe predetermined potential.

With this invention, a similar operational effect as in the firstinvention can be attained.

According to a fifth invention, the fourth invention further includes aconstant voltage source outputting a constant voltage equal to thepredetermined potential, and a switching means, whose input can beswitched between the output voltage of the D/A converter and the outputvoltage of the constant voltage source, and which outputs one of thosetwo output voltages to the waveform generating portion, wherein theswitching means is configured such that when a digital signal forgenerating a voltage rising waveform or a voltage falling waveform withthe waveform generating portion is input into the D/A converter, theinput into the switching means is switched from the output voltage ofthe constant voltage source to the output voltage of the D/A converter.

Thus, a similar operational effect as in the second invention can beattained.

According to a sixth invention, in the fifth invention, the switchingmeans is configured such that the input into the switching means isswitched from the output voltage of the constant voltage source to theoutput voltage of the D/A converter after the output of the D/Aconverter has settled.

Thus, a similar operational effect as in the third invention can beattained.

A seventh invention is an invention of an inkjet recording device, andthis invention includes:

an inkjet head having a pressure chamber filled with ink, a nozzlelinked to the pressure chamber, and an actuator that is caused to ejectthe ink inside the pressure chamber through the nozzle by application ofa voltage;

a relative movement means that moves the inkjet head and a recordingmedium relatively to one another; and

an inkjet head driving circuit driving the actuator of the inkjet head;

wherein the inkjet head driving circuit comprises one D/A converter thatconverts a digital signal into an analog voltage and outputs the analogvoltage, and a waveform generating portion into which an output voltageof the D/A converter is input, which generates one of a voltage risingwaveform and a voltage falling waveform and outputs it to the actuatorwhen the output voltage of the D/A converter is larger than apredetermined potential that is midway between a maximum value and aminimum value of that output voltage, and which generates the otherwaveform and outputs it to the actuator when the output voltage of theD/A converter is smaller than the predetermined potential; and

wherein recording is performed by ejecting ink from the nozzle of theinkjet head onto the recording medium by outputting to the actuator thevoltage waveform generated by the waveform generating portion of theinkjet head driving circuit when the inkjet head is moved in relation tothe recording medium by the relative movement means.

With this invention, an inkjet recording device easily can be attained,which is compact, inexpensive and has superior ink ejection performance.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic perspective view showing an inkjet recordingdevice in accordance with an embodiment of the present invention.

FIG. 2 is a cross-sectional drawing taken along the main scan directionof the inkjet head of the inkjet recording device of FIG. 1.

FIG. 3 is a schematic circuit diagram showing a first example of aninkjet head driving circuit driving piezoelectric actuators for inkjetejection provided on an inkjet head.

FIG. 4 is a waveform diagram showing an example of a voltage waveformapplied to the piezoelectric actuators.

FIG. 5 is a time-chart for generating the voltage waveform in FIG. 4with the inkjet head driving circuit according to the first example.

FIG. 6 is a schematic circuit diagram showing a second example of aninkjet head driving circuit.

FIG. 7 is a time-chart for generating the voltage waveform in FIG. 4with the inkjet head driving circuit according to the second example.

FIG. 8 is a waveform diagram showing an example of a voltage waveformgenerated with a conventional inkjet head driving circuit.

FIG. 9 is a schematic circuit diagram showing a conventional inkjet headdriving circuit.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 schematically shows an inkjet recording device in accordance withan embodiment of the present invention. This inkjet recording device isprovided with an inkjet head H that ejects ink onto recording paper 51serving as a recording medium, as will be described later. The inkjethead H is fixed to and supported by a carriage 31, which is providedwith a carriage motor not shown in the drawings, by which the inkjethead H and the carriage 31 are guided on a carriage shaft 32 extendingin the primary scan direction (X direction in FIG. 1), moving back andforth in this direction. The carriage 31, the carriage shaft 32 and thecarriage motor constitute a relative movement means for moving theinkjet head H relatively to the recording paper 51.

The recording paper 51 is clamped by two feed rollers 52 that arerotatively driven by a feed motor not shown in the drawings. The feedmotor and the feed rollers 52 feed the recording paper 51 in a secondaryscan direction (Y direction in FIG. 1), which is perpendicular to theprimary scan direction, below the inkjet head H.

As shown in FIG. 2, the inkjet head H includes a head body 1, in which aplurality of pressure chamber cavities 2 are formed, which have a supplyport 2 a for supplying ink and an ejection port 2 b for ejecting ink.The cavities 2 of the head body 1, which are formed in the upper side ofthe head body 1, are extended in the primary scan direction, and arelined up in the secondary direction, leaving a substantially equalspacing between them.

The lateral wall portions of the cavities 2 of the head body 1 areconstituted by a pressure chamber component 5 made of photosensitiveglass of about 200 μm thickness, and the bottom wall portion of thecavities 2 is made of an ink channel component 6 that is affixed to thepressure chamber component 5 and made by laminating a plurality ofstainless steel sheets. Inside this ink channel component 6, supply inkchannels 7 connected to the supply ports 2 a of the cavities 2 andejection ink channels 8 connected to the ejection ports 2 b of thecavities 2 are formed. Each supply ink channel 7 is connected to an inksupply chamber 10 extending in the same direction in which the cavities2 are lined up (secondary scan direction). This ink supply chamber 10 isformed by the pressure chamber component 5 and the ink channel component6, and is connected to an ink supply hole 11 connected to an ink tankoutside the drawing.

On the side of the ink channel component 6 that is opposite to thepressure chamber component 5 (i.e. the lower side), a nozzle plate 13 ofabout 20 μm thickness is provided, which constitutes the lower side ofthe inkjet head H and is made of a polymer resin, such as polyimide.Nozzles 14 of about 20 μm diameter are formed in the nozzle plate 13,and are respectively connected to the ejection ports 2 b through theejection ink channel 8. The nozzles 14 are aligned in a row in thesecondary scan direction.

In the pressure chamber component 5 of the head body 1, the sideopposite the ink channel component 6 (i.e. the upper side) is providedwith piezoelectric actuators 21 that cover the cavities 2 of the headbody 1 and form the pressure chambers 3 together with the cavities 2.These piezoelectric actuators 21 include a piezoelectric layer 23 of 1to 10 μm thickness made of lead zirconium titanate (PZT), an upperelectrode layer 24 made of Pt of 0.05 to 0.6 μm thickness provided onthe side of the piezoelectric layer 23 that is opposite the pressurechamber 3 (i.e. the upper side), and a lower electrode layer 22 made ofCr of 1 to 10 μm thickness provided on the pressure chamber 3 side ofthe piezoelectric layer 23 (i.e. the lower side). The lower electrodelayer 22, which is shared by all piezoelectric actuators 21, is groundedand fulfills the function of a so-called oscillation plate.

FIG. 3 shows a first example of an inkjet head driving circuit (waveformgenerating circuit) driving the piezoelectric actuators 21 for inkejection provided in the inkjet head H. The inkjet head driving circuitaccording to this first example includes a CPU 61 having a terminaloutputting a digital signal (of for example 8 bits) for generating avoltage waveform, one D/A converter 62 connected to the digital signaloutput terminal of the CPU 61, which converts the digital signal into ananalog signal and outputs it, and a waveform generating portion 64 intowhich the output voltage (analog voltage) of the D/A converter 62 isinput, and which, based on this output voltage, generates a voltagewaveform as explained below and outputs it to the piezoelectric actuator21.

The D/A converter 62 receives from the CPU 61 a data set signal togetherwith the digital signal but from a different terminal than the digitalsignal, and when this data set signal has been input and a predeterminedtime (data settling time: depends on the output voltage) has elapsedafter its input (after the output of the D/A converter 62 has settled),the analog voltage is given out. The D/A converter 62 is connected to apower source 66 giving out a positive voltage V1, and is configured suchthat it can output any voltage from ground potential to the outputvoltage V1 of the power source 66, depending on the digital signal.

The output terminal of the D/A converter 62 is connected to the waveformgenerating portion 64. The waveform generating portion 64 includes avoltage/current converter 64 a, which converts the analog voltage outputfrom the D/A converter 62 into a current, and a current/voltageconverter amplifier 64 b, which amplifies, with a current mirror circuitmade of two resistors and two transistors, the current into which thevoltage has been converted by the voltage/current converter 64 a (theamplification ratio depending on the resistance ratio between the tworesistors), and which converts the amplified current into a voltage witha capacitor. The voltage/current converter 64 a and the current/voltageconverter amplifier 64 b are connected to the same power source 66 asthe D/A converter 62.

The waveform generating portion 64 is configured such that when theoutput voltage of the D/A converter 62 is larger than the predeterminedpotential V2, which is midway between the maximum value (V1) and theminimum value (ground potential) of this output voltage (in thisembodiment, the median potential (V1/2) of the maximum value and theminimum value is taken, but any value is suitable that is midway betweenthe maximum value and the minimum value), then the waveform generatingportion 64 generates a voltage rising waveform, whereas when the outputvoltage of the D/A converter 62 is smaller than the predeterminedpotential V2, then the waveform generating portion 64 generates avoltage falling waveform. Moreover, when the output voltage of the D/Aconverter 62 is the predetermined potential V2, then it generates awaveform maintaining the potential directly before the output of thatpredetermined potential V2.

The output terminal of the waveform generating portion 64 is connectedvia a current amplifier 68, which is made of two transistors 68 a, and adriver IC 69 to the upper electrode layer 24 of the piezoelectricactuators 21 of the inkjet head H. The driver IC 69 includes for exampleswitching transistors provided in correspondence with the piezoelectricactuators 21, and, receiving print signals from the CPU 61, selects thepiezoelectric actuators 21 corresponding to the nozzles 14 through whichink drops are to be ejected, thus applying the voltage waveform that isgenerated and output by the waveform generating portion 64 only to theselected actuators 21.

As shown for example in FIG. 4, the voltage waveform applied to thepiezoelectric actuators 21 is made of a first waveform P1 (voltagefalling waveform), at which the potential falls from an intermediatepotential Vb, which is midway between the maximum potential (Va) and theminimum potential (ground potential), to the minimum potential, a secondwaveform P2 continuing the first waveform P1 and maintaining thisminimum potential, a third waveform P3 (voltage rising waveform)continuing the second waveform P2 and rising from the minimum potentialto the maximum potential, a fourth waveform P4 continuing the thirdwaveform P3 and maintaining this maximum potential, and a fifth waveformPS (voltage falling waveform) continuing the fourth waveform P4 andreturning from the maximum potential to the intermediate potential Vb.This series of first to fifth waveforms P1 to P5 constitutes one drivingpulse P for ejecting one ink drop from the nozzle 14, and the drivingpulse P is given out repeatedly with a predetermined period (for exampleabout 50 μs, i.e. a driving frequency of 20 kHz). The potential betweenneighboring driving pulses P is maintained at the intermediate potentialVb. That is to say, the driving pulse P is of the pull-push-pull typewith the intermediate potential Vb as the reference potential.

Next, the operation of this first example of an inkjet head drivingcircuit for generating these first to fifth waveforms P1 to P5 isexplained with FIG. 5.

At the stage before generating the waveforms, a digital signal, whichsets the output voltage of the D/A converter 62 to the predeterminedpotential V2, is given out from the CPU 61 to the D/A converter 62.Thus, the waveform generating portion 64 outputs the intermediatepotential Vb.

Then, a data set signal and a digital signal that sets the outputvoltage of the D/A converter 62 to a lower value than the predeterminedpotential V2 (in this embodiment, to the minimum value of the outputvoltage (ground potential)) are output from the CPU 61 to the D/Aconverter 62 (in FIG. 5, the Lo state is assumed during the output ofthe data set signal), and after a predetermined time (the time t in FIG.5) has elapsed after the input of the data set signal, the output of theD/A converter 62 is settled, and the analog signal into which thedigital signal has been converted is output (in FIG. 5, the Lo state isassumed during the output of the analog signal (i.e. outside thepredetermined potential V2), and the Hi state is assumed during theoutput of the predetermined potential V2). Due to the output of thisanalog voltage, the waveform generating portion 64 generates/outputs,with its current/voltage converter amplifier 64 b, the first waveformP1, at which the potential falls from the intermediate potential Vb tothe minimum potential. It should be noted that before the output of theD/A converter 62 has settled, the driver IC 69 receives print signalsfrom the CPU 61 to select the piezoelectric actuators 21 correspondingto the nozzles 14 through which ink drops are to be ejected, and setsthe switching transistors corresponding to the selected piezoelectricactuators 21 to the ON state, which is then maintained until after thegeneration of the fifth waveform P5 has been terminated.

Subsequently, after the generation of the first waveform P1 has beenterminated, a digital signal that sets the output voltage of the D/Aconverter 62 to the predetermined potential V2 is output from the CPU 61to the D/A converter 62, so that the waveform generating portion 64generates/outputs the second waveform P2, at which the minimum potentialis maintained.

Next, a data set signal and a digital signal that sets the outputvoltage of the D/A converter 62 to a larger value than the predeterminedpotential V2 (in this embodiment, to the maximum value V1 of the outputvoltage) are output from the CPU 61 to the D/A converter 62, and whenthe output of the D/A converter 62 has settled, the analog voltage isoutput. Due to the output of this analog voltage, the waveformgenerating portion 64 generates/outputs, with its current/voltageconverter amplifier 64 b, the third waveform P3, at which the potentialrises from the minimum potential to the maximum potential.

Then, after the generation of the third waveform P3 has been terminated,a digital signal that sets the output voltage of the D/A converter 62 tothe predetermined potential V2 is output from the CPU 61 to the D/Aconverter 62, so that the waveform generating portion 64generates/outputs the fourth waveform P4, at which the maximum potentialis maintained.

Next, a data set signal and a digital signal that sets the outputvoltage of the D/A converter 62 to the minimum value are output from theCPU 61 to the D/A converter 62, and when the output of the D/A converter62 has settled, the analog voltage is output. Due to the output of thisanalog voltage, the waveform generating portion 64 generates/outputs,with its current/voltage converter amplifier 64 b, the fifth waveformP5, at which the potential falls from the maximum potential to theintermediate potential.

Then, after the generation of the fifth waveform P5 has been terminated,a digital signal that sets the output voltage of the D/A converter 62 tothe predetermined potential V2 is output from the CPU 61 to the D/Aconverter 62, so that the waveform generating portion 64 outputs theintermediate potential Vb, until the next driving pulse P is generated.

The operation of the inkjet head H is as follows: when the firstwaveform P1 generated/output by the waveform generating portion 64 isapplied to the piezoelectric actuator 21, the piezoelectric layer 23expands in the direction perpendicular to its thickness direction, dueto the electric field created inside the piezoelectric layer 23, whereasthe lower electrode layer 22 and the upper electrode layer 24 do notexpand, so that due to the so-called bi-metal effect, a portion of thepiezoelectric actuator 21 that corresponds to the pressure chamber 3 isdeformed and bent such that it becomes convex on the side opposite thepressure chamber 3.

Then, when the third waveform P3 is applied to the piezoelectricactuator 21, the piezoelectric layer 23 contracts, and the portion ofthe piezoelectric actuator 21 that corresponds to the pressure chamber 3is deformed and bent such that it becomes convex on the side of thepressure chamber 3. This bend deformation causes a pressure inside thepressure chamber 3, and due to this pressure, a predetermined amount ofthe ink in the pressure chamber 3 is ejected via the ejection port 2 band the ejection ink channel 8 and through the nozzle 14 onto therecording paper 51, adhering in dot shape to the recording paper 51.

Next, when the fifth waveform P5 is applied to the piezoelectricactuator 21, the piezoelectric layer 23 expands, and the portion of thepiezoelectric actuator 21 that corresponds to the pressure chamber 3 isreturned to its original state. During the application of the first andthe fifth waveforms P1 and P5, ink is filled from the ink supply chamber10 via the supply ink channel 7 and the supply port 2 a into thepressure chamber 3.

The application of the voltage waveform to the piezoelectric actuator 21is carried out repeatedly at an output period of the driving pulse Pwhile the inkjet head H and the carriage 31 are moved at substantiallyconstant speed in the primary scan direction from one end of therecording paper 51 to the other (however, when the inkjet head H hasreached a location on the recording paper 51 onto which no ink drop isshot, the voltage waveform is not applied by the driver IC 69), and thusink drops are shot onto predetermined positions of the recording paper51. Then, when the recording for one scan line has been finished, therecording paper 51 is fed by the feed motor and the feed rollers 52 fora predetermined amount in the secondary scan direction, and ink dropsare ejected again while the inkjet head H and the carriage 31 are movedin the primary scan direction, and the recording for the next scan lineis carried out. By repeating this operation, the desired image is formedover the entire recording paper 51.

Consequently, with the inkjet head driving circuit according to thisfirst example, a voltage rising and a voltage falling waveform aregenerated taking as a reference the predetermined potential V2, which ismidway between the maximum value and the minimum value of the outputvoltage of one D/A converter 62, so that two D/A converters, as in theconventional circuit, are not necessary, and moreover, a power sourceoutputting a negative voltage is not necessary and one power source 66outputting a positive voltage is sufficient. Furthermore, waveformgeneration discrepancies due to discrepancies in the characteristicsamong the two D/A converters (discrepancies among variation amounts), asin the conventional circuit, do not occur. As a result, it is possibleto achieve a circuit that is less expensive and uses less space, andstable voltage waveforms can be generated. Thus, an inkjet recordingdevice that is compact and inexpensive and has superior ink ejectionperformance can be easily attained.

FIG. 6 shows an inkjet head driving circuit in accordance with a secondexample (portions that are the same as in FIG. 3 have been denoted byidentical numerals and their further explanation has been omitted),which has an analog switch 71 serving as a switching means providedbetween the D/A converter 62 and the waveform generating portion 64.

That is to say, in this second example, a constant voltage source 72that outputs a constant voltage equal to the predetermined potential V2is provided, and the analog switch 71 outputs either the output voltageof the D/A converter 62 or the output voltage of the constant voltagesource 72 to the waveform generating portion 64, its switching statebeing determined by an operation signal from the CPU 61. Morespecifically, when neither a voltage rising waveform nor a voltagefalling waveform is being generated by the waveform generating portion64 (when no digital signal for generating a voltage rising waveform or avoltage falling waveform with the waveform generating portion 64 isbeing input to the D/A converter 62), then the input into the analogswitch 71 is the output voltage of the constant voltage source 72 (theanalog switch 71 is set to the state indicated by the solid line in FIG.6), and when a digital signal for generating either a voltage risingwaveform or a voltage falling waveform with the waveform generatingportion 64 is input to the D/A converter 62 (when a digital signalsetting the output voltage of the D/A converter 62 to the maximum valueor the minimum value is input from the CPU 61 to the D/A converter 62),then the input into the analog switch 71 is switched from the outputvoltage of the constant voltage source 72 to the output voltage of theD/A converter 62 (the analog switch 71 is set to the state indicated bythe double-dashed line in FIG. 6). This switching of the input isperformed after the digital signal for generating a voltage risingwaveform or a voltage falling waveform with the waveform generatingportion 64 has been input and the output of the D/A converter 62 hassettled.

It should be noted that a latch signal is input into the D/A converter62 through a terminal different from that for the digital signal and thedata set signal, and the output state of the analog voltage ismaintained by the input of this latch signal.

Next, the operation of the inkjet head driving circuit according to thissecond example for generating the first to fifth waveforms P1 to P5 isexplained with FIG. 7.

At the stage before waveform generation, the analog switch 71 receivesan operation signal (Hi state in FIG. 7) from the CPU 61, which puts itinto a state connecting the constant voltage source 72 and the waveformgenerating portion 64, and thus the output voltage of the constantvoltage source 72 is input into the waveform generating portion 64. Theoutput voltage of the constant voltage source 72 is equal to thepredetermined potential V2, so that as in the first example, thewaveform generating portion 64 outputs the intermediate potential Vb.

Next, a data set signal and a digital signal that sets the outputvoltage of the D/A converter 62 to the minimum value are output from theCPU 61 to the D/A converter 62, and after a predetermined time haselapsed after the input of the data set signal, the output of the D/Aconverter 62 is settled, and the analog voltage is output. This outputstate is maintained by the above-mentioned latch signal.

Then, after the output of the D/A converter 62 has settled, and theanalog voltage has been output (after a time that is slightly longerthan the maximum value of the data settling time has elapsed after theinput of the data set signal), the input into the analog switch 71 isswitched, with an operation signal from the CPU 61 (Lo state in FIG. 7),from the output voltage of the constant voltage source 72 to the outputvoltage of the D/A converter 62. Thus, the D/A converter 62 and thewaveform generating portion 64 become connected, and the analog voltageis input into the waveform generating portion 64, and as a result, thewaveform generating portion 64 generates/outputs the first waveform P1,as in the first example described above. It should be noted that, inthis second example, substantially at the same time as the switching ofthe input into the analog switch 71, the driver IC 69 sets the switchingtransistors corresponding to the selected piezoelectric actuators 21 tothe ON state, and this state is continued substantially until the end ofthe generation of the fifth waveform P5.

After that, at substantially the same time as the end of the generationof the first waveform P1, the input into the analog switch 71 isswitched from the output voltage of the D/A converter 62 to the outputvoltage of the constant voltage source 72, and thus the waveformgenerating portion 64 generates/outputs the second waveform P2. Itshould be noted that substantially at the same time as the end of thegeneration of the first waveform P1 or after that, the maintaining ofthe output state of the D/A converter 62 due to the latch signal isterminated.

Subsequently, a data set signal and a digital signal that sets theoutput voltage of the D/A converter 62 to the maximum value are outputfrom the CPU 61 to the D/A converter 62, and when the output of the D/Aconverter 62 has settled, the analog voltage is output. Then, after theoutput of the D/A converter 62 has settled, and the analog voltage hasbeen output (after a time that is slightly longer than the maximum valueof the data settling time has elapsed after the input of the data setsignal), the input into the analog switch 71 is switched from the outputvoltage of the constant voltage source 72 to the output voltage of theD/A converter 62. Thus, the D/A converter 62 and the waveform generatingportion 64 become connected, the analog voltage is input into thewaveform generating portion 64, and the waveform generating portion 64generates/outputs the third waveform P3.

After that, at substantially the same time as the end of the generationof the third waveform P3, the input into the analog switch 71 isswitched from the output voltage of the D/A converter 62 to the constantvoltage source 72, and thus the waveform generating portion 64generates/outputs the fourth waveform P4.

Subsequently, a data set signal and a digital signal that sets theoutput voltage of the D/A converter 62 to the minimum value are outputfrom the CPU 61 to the D/A converter 62, and when the output of the D/Aconverter 62 has settled, the analog voltage is output. Then, after theoutput of the D/A converter 62 has settled, and the analog voltage hasbeen output (after a time that is slightly longer than the maximum valueof the data settling time has elapsed after the input of the data setsignal), the input into the analog switch 71 is switched from the outputvoltage of the constant voltage source 72 to the output voltage of theD/A converter 62. Thus, the waveform generating portion 64generates/outputs the fifth waveform P5.

Subsequently, at substantially the same time as the end of thegeneration of the fifth waveform P5, the input into the analog switch 71is switched from the output voltage of the D/A converter 62 to theoutput voltage of the constant voltage source 72, and thus the waveformgenerating portion 64 outputs the intermediate potential Vb until thenext driving pulse P is generated.

It should be noted that during the time in which the output voltage ofthe constant voltage source 72 is being input into the analog switch 71,the output voltage of the D/A converter 62 can be the predeterminedpotential V2 as in the first example, or another potential, such asground potential.

Consequently, with the inkjet driving circuit according to the secondexample, when neither a voltage rising waveform or a voltage fallingwaveform is being generated (when no digital signal for generating avoltage rising waveform or a voltage falling waveform with the waveformgenerating portion 64 is being input into the D/A converter 62), theoutput voltage of the constant voltage source 72 serves as the inputinto the analog switch 71, so that the predetermined potential V2 fromthe constant voltage source 72 is output to the waveform generatingportion 64, regardless of the output voltage of the D/A converter 62.That is to say, even if a digital signal setting the output voltage ofthe D/A converter 62 to the predetermined potential V2 is output by theCPU 61 to the D/A converter 62, there is the possibility that the outputof the D/A converter 62 deviates slightly from the predeterminedpotential V2 due to variations in the characteristics of the D/Aconverter 62, but in this second example, the predetermined potential V2is output from the constant voltage source 72, which can output aprecise voltage, to the waveform generating portion 64, and thus,malfunctioning of the waveform generating portion 64 due to variationsin the characteristics of the D/A converter 62 can be prevented. As aconsequence, the ink ejection performance of the inkjet recording devicecan be improved.

Furthermore, when a voltage rising waveform or a voltage fallingwaveform is generated by the waveform generating portion 64 (when adigital signal for generating a voltage rising waveform or a voltagefalling waveform with the waveform generating portion 64 is input intothe D/A converter 62), then the input into the analog switch 71 isswitched from the output voltage of the constant voltage source 72 tothe output voltage of the D/A converter 62, and in this situation, afterthe output of the D/A converter 62 has settled, the input into theanalog switch 71 is switched, so that the waveform generation timing canbe controlled with the analog switch 71. That is to say, the time(predetermined time t) from the input of the data set signal until theoutput of the D/A converter 62 has settled fluctuates depending on theoutput voltage of the D/A converter 62 and variations in itscharacteristics, so that if there is no analog switch 71, or even ifthere is the analog switch 71, but the input into the analog switch 71is switched to the output voltage of the D/A converter 62 before theoutput of the D/A converter 62 has settled, then the result isvariations in the generation/output timing of the voltage risingwaveform or the voltage falling waveform by the waveform generatingportion 64. However, in this second example, the input into the analogswitch 71 is switched to the output voltage of the D/A converter 62 onlyafter the output of the D/A converter 62 has settled, so that thevoltage rising waveform or voltage falling waveform can begenerated/output substantially at the same time as the switching of theinput into the analog switch 71. As a result, variations in the waveformgenerating timing brought about by fluctuations in the output settlingtime of the D/A converter 62 are prevented, and voltage waveforms can begenerated/output at an ordinarily constant timing. Thus, variations inthe ink ejection amount can be suppressed to a rather small amount, andthe positional precision with which ink drops are shot onto therecording paper 51 can be improved, attaining a high image quality.

It should be noted that in the second example, the input into the analogswitch 71 is switched from the output voltage of the constant voltagesource 72 to the output voltage of the D/A converter 62 after the outputof the D/A converter 62 has settled, but it is also possible to switchthe input into the analog switch 71 to the output voltage of the D/Aconverter 62 before the output of the D/A converter 62 has settled (forexample, at the same time as the digital signal for generating thevoltage rising waveform or the voltage falling waveform with thewaveform generating portion 64 is input into the D/A converter 62). Alsoin this case, malfunctioning of the waveform generating portion 64 dueto variations in the characteristics of the D/A converter 62 can beprevented. However, for example with regard to improving the positionalprecision with which the ink drops are shot, it is preferable that thesecond example is adopted.

Furthermore, in the above embodiments, the driving pulse P is of thepull-push-pull type, but the present invention can also be applied topush-pull types or pull-push types having only one voltage risingwaveform and one voltage falling waveform.

Moreover, in the above embodiments, the waveform generating portion 64is configured such that it generates a voltage rising waveform when theoutput voltage of the D/A converter 62 is larger than a predeterminedpotential V2, and a voltage falling waveform when the output voltage ofthe D/A converter 62 is smaller than the predetermined potential V2, butit can also be configured such that it generates the voltage fallingwaveform when the output voltage of the D/A converter 62 is larger thana predetermined potential V2, and a voltage rising waveform when theoutput voltage of the D/A converter 62 is smaller than the predeterminedpotential V2.

Furthermore, in the above embodiments, the waveform generating circuitwas applied to an inkjet head driving circuit driving the piezoelectricactuators 21 of an inkjet head H in an inkjet recording device, but thewaveform generating circuit of the present invention can be applied toany device that drives by applying a voltage pulse having a voltagerising waveform and a voltage falling waveform.

INDUSTRIAL APPLICABILITY

The present invention is useful for devices on which actuators aremounted that are driven by application of voltage pulses, in particularfor inkjet recording devices provided with actuators for ink ejection,and its industrial applicability is high in that it can achieve acircuit that is less expensive and takes up less space, and stablevoltage waveforms can be generated.

What is claimed is:
 1. A waveform generating circuit, comprising: oneD/A converter that converts a digital signal into an analog voltage andoutputs the analog voltage; and a waveform generating portion into whichan output voltage of the D/A converter is input, which generates one ofa voltage rising waveform and a voltage falling waveform when the outputvoltage of the D/A converter is larger than a predetermined potentialthat is midway between a maximum value and a minimum value of thatoutput voltage, and generates the other waveform when the output voltageof the D/A converter is smaller than the predetermined potential.
 2. Thewaveform generating circuit according to claim 1, further comprising: aconstant voltage source outputting a constant voltage equal to thepredetermined potential; and a switching means, whose input can beswitched between the output voltage of the D/A converter and the outputvoltage of the constant voltage source, and which outputs one of thosetwo output voltages to the waveform generating portion; wherein theswitching means is configured such that when a digital signal forgenerating a voltage rising waveform or a voltage falling waveform withthe waveform generating portion is input into the D/A converter, theinput into the switching means is switched from the output voltage ofthe constant voltage source to the output voltage of the D/A converter.3. The waveform generating circuit according to claim 2, wherein theswitching means is configured such that the input into the switchingmeans is switched from the output voltage of the constant voltage sourceto the output voltage of the D/A converter after the output of the D/Aconverter has settled.
 4. An inkjet head driving circuit driving anactuator for ink ejection provided on an inkjet head, comprising: oneD/A converter that converts a digital signal into an analog voltage andoutputs the analog voltage; and a waveform generating portion into whichan output voltage of the D/A converter is input, which generates one ofa voltage rising waveform and a voltage falling waveform and outputs itto the actuator when the output voltage of the D/A converter is largerthan a predetermined potential that is midway between a maximum valueand a minimum value of that output voltage, and which generates theother waveform and outputs it to the actuator when the output voltage ofthe D/A converter is smaller than the predetermined potential.
 5. Theinkjet head circuit according to claim 4, further comprising: a constantvoltage source outputting a constant voltage equal to the predeterminedpotential; and a switching means, whose input can be switched betweenthe output voltage of the D/A converter and the output voltage of theconstant voltage source, and which outputs one of those two outputvoltages to the waveform generating portion; wherein the switching meansis configured such that when a digital signal for generating a voltagerising waveform or a voltage falling waveform with the waveformgenerating portion is input into the D/A converter, the input into theswitching means is switched from the output voltage of the constantvoltage source to the output voltage of the D/A converter.
 6. Thewaveform generating circuit according to claim 5, wherein the switchingmeans is configured such that the input into the switching means isswitched from the output voltage of the constant voltage source to theoutput voltage of the D/A converter after the output of the D/Aconverter has settled.
 7. An inkjet recording device, comprising: aninkjet head having a pressure chamber filled with ink, a nozzle linkedto the pressure chamber, and an actuator that is caused to eject the inkinside the pressure chamber through the nozzle by application of avoltage; a relative movement means that moves the inkjet head and arecording medium relatively to one another; and an inkjet head drivingcircuit driving the actuator of the inkjet head; wherein the inkjet headdriving circuit comprises one D/A converter that converts a digitalsignal into an analog voltage and outputs the analog voltage, and awaveform generating portion into which an output voltage of the D/Aconverter is input, which generates one of a voltage rising waveform anda voltage falling waveform and outputs it to the actuator when theoutput voltage of the D/A converter is larger than a predeterminedpotential that is midway between a maximum value and a minimum value ofthat output voltage, and which generates the other waveform and outputsit to the actuator when the output voltage of the D/A converter issmaller than the predetermined potential; and wherein recording isperformed by ejecting ink from the nozzle of the inkjet head onto therecording medium by outputting to the actuator the voltage waveformgenerated by the waveform generating portion of the inkjet head drivingcircuit when the inkjet head is moved in relation to the recordingmedium by the relative movement means.